[XFRM] IPV6: Optimize __xfrm_tunnel_alloc_spi().
[XFRM] IPV6: Optimize __xfrm_tunnel_alloc_spi().

| % size old/net/ipv6/xfrm6_tunnel.o new/net/ipv6/xfrm6_tunnel.o
|    text	   data	    bss	    dec	    hex	filename
|    1606	     40	   2080	   3726	    e8e	old/net/ipv6/xfrm6_tunnel.o
|    1574	     40	   2080	   3694	    e6e	new/net/ipv6/xfrm6_tunnel.o

Signed-off-by: YOSHIFUJI Hideaki <948b80da4254122ea1e8973dd9930d46b7654cf3@linux-ipv6.org>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas